This invention relates, in general, to coherent radio frequency memories (CRFM) and, more specifically, to digital radio frequency memories (DRFM) suitable for use in radar countermeasures equipment.
Active jammers are used in the field of electronic countermeasures to confuse or counter the system originating radar signals. In some situations, it is desirable to return signals to the radar system which are exact copies of the arriving radar signal. In other situations, it is desirable to return signals to the radar system which have characteristics other than that of the received radar signal in order to further confuse the radar system. In any event, it is usually necessary for the countermeasures system to store the received radar signal and reproduce it at a later time.
Previously, delay lines of various types have been used to effectively store the received radar signal for a short period of time and make the stored radar signal available at a later time. One disadvantage of a delay line is that the delay cannot be electronically changed easily. It is also difficult to obtain reasonably long delay periods because the delay line signal degrades and the equipment is bulky. An improvement over the delay line technology has been achieved by the use of digital radio frequency memories (DRFMs) which convert relatively high radio frequency (RF) signals down to a lower intermediate frequency (IF) by mixing the RF with a local oscillator (LO) signal for storage in a digital memory device. The digital memory can be controlled in a manner similar to the digital memory of a computer and the stored value representing the radar signal can be recalled at RF at any time delay desired. Further, manipulation of the digital values to produce changes in the replicated signal are also conveniently done by the digital processes.
Digital systems work by sampling the incoming signal at a selected sampling rate. The higher the frequency of the incoming signal the higher sampling rate necessary to fully characterize the signal in digital form. The maximum usable instantaneous bandwidth (IBW) of a DRFM is one-half the sampling rate of the sampling devices, e.g., analog-to-digital (A/D) and digital-to-analog (D/A) converters used in the DRFM to move into and out of the digital domain, based upon the limitations governed by the Nyquist Sampling Theory.
Typically, to improve the amplitude dynamic range of the DRFM, to maximize instantaneous bandwidth and to minimize the digital storage bit requirement needed, one bit sampling and replication techniques are used in most DRFMs. One bit sampling involves sampling the incoming signal to determine the signal polarity (positive or negative) at each sample point. However, even with the instantaneous bandwidth provided by one bit sampling it is possible that the incoming signal may have a frequency spectrum which exceeds the instantaneous bandwidth of the DRFM. When this happens part of the incoming signal is lost. In order to handle a wider operating frequency range it is possible to employ multiple down conversion local oscillators. These oscillators provide the local oscillator signal near the frequency of the incoming signal. However, once the local oscillator is selected it is not switched until the next incoming pulse. Thus, if the operating frequency of the incoming pulse is wide, part of another signal may be lost.